Funny you ask this...I just started pumping again for my 9 month old and was wondering the same thing...what I've found, which you can check on kellymom.com & drsears' website. Is that freshly expressed breast milk can sit at room temp (as previous posters have mentioned) from 6-10hrs, conservative estimate is 4 hours. If you've started feeding and there is still some left over in the bottle they say you can refrigerate and it has to be used in the next four hours.
I however just use the taste and smell method personally. If I've given him freshly expressed breast milk (meaning it was never refrigerated) I put back in the fridge and just toss it by the end of the day if it wasn't used. If its been warmed or thawed and not finished I will only use it for the next feeding, if its not used at that point it's tossed. I remember reading somewhere that cows milk (pasteurized even I think) can be okay to drink for up to 24 hrs after drinking some. So my thoughts are that BM has far more antibacterial properties than that....so I'm more apt to reuse BM if it doesn't fit the 4-6 hr mark...
However, with such a young baby I think I'd err on the side of caution and just store in smaller increments and be conservative on reusing the BM.. What I do when I have a lot of milk (not very common these days) is store in the Mama's milk bags and pour from there, the amount I think he's going to need. If he needs more then pour more. But then again my pumping success has been limited these days. At the beginning I could care less, it was like I was a magic fountain and I was just over flowing with the good stuff!!
Good Luck if you have any questions for sure I'm sure your pediatrician and or lactation consultant can give you some direct advice :)